1 / 26

Chapter 5 Evolution & Gene Frequencies

Chapter 5 Evolution & Gene Frequencies. Populations & Gene Pools. Evolution-def -described as any change in the frequency of alleles, & resulting phenotypes, in a population. Population-def- consists of the animals in a particular place that could interbreed

Download Presentation

Chapter 5 Evolution & Gene Frequencies

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Chapter 5Evolution & Gene Frequencies

  2. Populations & Gene Pools • Evolution-def-described as any change in the frequency of alleles, & resulting phenotypes, in a population. • Population-def-consists of the animals in a particular place that could interbreed • Populations evolve as gene frequencies change over time • Change in frequency of alleles in gene pool indicates the presence of evolutionary change

  3. Hardy-Weinberg Theorem • Hardy-Weinberg theorem-states that when certain assumptions are met, the frequency of alleles in a population will not change over time = no evolution • 4 assumptions of Hardy-Weinberg Theorem: • Population size must be large to prevent the change in allelic frequencies by chance alone • Mating must be random • Migration must not occur, as it would add/delete alleles from gene pool • Mutation must not occur, or mutational equilibrium must exist

  4. Hardy-Weinberg Theorem • The assumptions of the Hardy-Weinberg theorem are not typically met in natural populations most populations are evolving • Neutral Changes-Some of the features that might be changing which have no advantage to the organisms

  5. Evolutionary Mechanisms-Population size & Genetic Drift & Neutral Evolution • Evolution can result in some individual surviving & being more effective at reproducing than others in a population • The smaller a pop. size  the more significant chances of change to occur • Genetic drift-def-chance events influencing the frequencies of genes in a population • Neutral Evolution-def-gene frequencies change independent of natural selection & b/c of this genetic drift is neutral evolution

  6. Evolutionary Mechanisms-Population size & Genetic Drift & Neutral Evolution • Genetic drift is like flipping a coin: • Lrg sample = closer to 50:50 ration • Sm Sample can =: • Unusual proportions of alleles due to randomness • Inbreeding can be common •  genetic drift & inbreeding will likely reduce genetic variation w/in pop. • Mutations & genetic drift • If a mutation of an allele gets introduced into a pop. & it doesn’t make the allele more or less adaptive then the new allele could be: • Established in the pop. • Or it could be lost in the pop. due to genetic drift • If genetic drift can occur in sm. pop. Then Hardy-Weinberg equilibrium can’t happen

  7. Evolutionary Mechanisms-Population size & Genetic Drift & Neutral Evolution • Special Cases of Genetic Drift: • Founder Effect • Founder effect-def-new pop. emerges from founding individual(s) are more likely to have a distinct genetic make-up w/ less variation in the pop. than a lrg’er pop. • Founder effect is seen when a sm. Subpop. Fragments from the main pop. & colonizes new habitat • Often seen on islands & previous uninhabited habitats • Ex/ the Afrikaner population  Huntington’s disease • Ex/Amish Community  polydactyly • Ex/Pingelan island community  total colorblindness

  8. Evolutionary Mechanisms-Population size & Genetic Drift & Neutral Evolution • Special Cases of Genetic Drift: • Bottleneck Effect-def-pg.69-changes in gene frequency that result when numbers in a population are drastically reduced as a result of the population being built up again from relatively few surviving individuals • Ex/Cheetah populations in South & East Africa • Ex/elephant seal in late 1800s • Increase numbers now however low genetic variability • Ex/ Human intentions are to revive endangered populations of organisms. Where can you see this becoming a problem?

  9. Evolutionary Mechanisms-Gene Flow & Mutations • Gene flow-def-pg71- changes in relative allelelic frequencies from migration of inidividuals • Individuals will immigrate into a population • Individuals will emigrate out of a population  Hardy-Weinberg theorem assumptions don’t apply &  populations are evolving • Gene Flow Effects can be different: • Increase in Gene flow between 2 populations =s change in a population • Ex/island & continental population can affect the genetic make-up of both populations  eventually leading to genetic make-up becoming similar • Lack of Gene flow between 2 populations will make changes in a population be less likely • Ex/ African elephants-tropical forest elephants vs. savannah elephants

  10. Evolutionary Mechanisms-Gene Flow & Mutations • Mutations • Source of variation that can prove adaptative for organisms • Counters loss of genetic material from genetic drift & natural selection • Increase probability that variations will be present to allow future generations to survive shocks to the environment • Mutations make extinction less likely • Mutations are random events& aren’t affect by mutations’ usefulness • Organism’s can filter out good mutations from bad ones • Most mutations are deleterious • Depending on the environment can be harmful/neutral • Mutational equilibrium-know this concept • It rarely happens • Mutation pressure-a measure of the tendency for gene frequencies to change through mutations

  11. Natural Selection Reexamined-Mode of Selection • Selection pressure-tendency for natural selection to occur & upset the Hardy-Weinberg Equilibrium • Modes of Selection: • Many phenotypes are spread out over bell shaped curve • Natural selection can affect a range of phenotypes in (3) ways: • Directional selection • Stabilizing selection • Disruptive selection

  12. Natural Selection Reexamined-Mode of Selection • Modes of Selection: • Directional Selection- occurs when individuals at one phenotypic extreme are at a disadvantage compared to all other individuals in the population • Deleterious genes decrease in frequency & all other genes increase in frequency • Can happen when • mutation gives rise to new gene • Environment changes to select against a phenotype • Ex/ Industrial Melanism

  13. Natural Selection Reexamined-Mode of Selection • Modes of Selection: • Disruptive Selection- circumstances selecting against individual of an intermediate phenotype • Produces distinct subpopulations • Ex/snails of (2) colors in tidepools • Stabilizing Selection-when both phenotypic extremes are deleterious this leads to narrowing of the phenotypic range • Ex/ horseshoe crab- found on the Atlantic Coast

  14. Balanced Polymorphism • Polymorphism-occurs in a population when 2 or more distinct forms exist w/o a range of phenotypes between them. • Balanced Polymorphism-occurs when different phenotypes are maintained at relatively stable frequencies in the population & may resemble a population in which disruptive selection operates

  15. Heterozygote Superiority • What is heterozygote superiority? • When the heterozygote is more fit than the either homozygous organism to survive in the given environment. • This can lead to balanced polymorphism which can lead to speciation • Ex/Sickle Cell anemia

  16. Species & Speciation • Fundamental unit of classification= species • Taxonomists classify species based on: • Similarities • differences • Species-a group of population in which genes are actually & potentially exchanged through interbreeding • This definition causes taxonomists problems: • Morphological characteristics • Reproductive criterion must be assumed based on morphological & ecological information • Fossil material

  17. Species & Speciation • Taxonomists generally incorporate the following into their categorization: • Morphology Criterion • Physiology Criterion • Embryology Criterion • Behavioral Criterion • Molecular Criterion • Ecological Criterion • What is speciation? • The formation of a new species • Only happens when a subpopulation can’t interbreed • when gene flow doesn’t happen between population & subpopulation

  18. Species & Speciation • How can speciation happen? • Reproductive isolation-def-when a populations are reproductively isolated, natural selection & genetic drift can result in evolution taking a different course in each subpopulation. • Types of Reproductive Isolation: • Premating Isolation: • Impenetrable barriers • Different mating behavior • Different breeding periods • Different habitats

  19. Species & Speciation • Types of Reproductive Isolation: • Post mating Isolation- prevents successful fertilization & development even though mating can occur: • Hybrids-usually sterile • Mismatched chromosomes • Developmental failures of fertilized egg & embyro -Types of speciation: • Allopatric speciation-def-occurs when subpopulation become geographically isolated from one another • Most common type of speciation • Ex/ Galapagos Finches • Combined forces of natural selection, mutation, isolation

  20. Species & Speciation • Types of Speciation • Parapatric speciation-def-pg75-occurs in small, local population called demes • Demes-areas that are not completely isolated from each other • Members w/in demes experience different selection pressures  speciation can occur • i.e. tidepools, ponds,etc. • This is theoretical & has not been observed  no known examples • Individuals w/in demes more likely to reproduce with each other than those outside of demes

  21. Species & Speciation • Types of Speciation: • Sympatric speciation- speciation that occurs w/in a single population in which a new species develops when members of a population develop a genetic difference that prevents then from reproducing w/members of original species • Happens most often in plant species

  22. Rates of Evolution • Phyletic Evolution-def-pg75-the idea that evolutionary changes occur at a slow, constant pace over millions of years • Periods of stasis = equilibrium which  result in stabilizing selection • Periods of stasis (a.k.a.equilibrium) can be interrupted by geological/climate/habitat change • These changes can cause some evolutionary changes to happen rapidly • These cause disruptive & directional selection to occur • These rapid changes “punctuates” the equilibrium  results in the Punctuated equilibrium model

  23. Rates of Evolution • Punctuated Equilibrium Model-def-pg76-long periods of stasis interrupted by brief periods of change • Rapid evolutionary changes have been observed in sm populations • Ex/pest acquiring resistance to pesticides • Ex/ bacteria acquiring resistance to antibiotics • The punctuated equilibrium model is can be used to explain the gaps in the fossil records between organisms that may not have a transitional stage

  24. Rates of Evolution • Molecular evolution & Gene duplication • Molecular evolution-def-involves all evolutionary changes which results from changes in the base sequence in DNA and/or the amino acids sequence in proteins • Scientists study the base sequences & protein sequences of organisms to see if they are highly conserved (closer evolutionary relationship) vs. not highly conserved (further evolutionary relationship) • But scientists compare many proteins or genes • Ex/ Cytochrome c

  25. Rates of Evolution • Molecular Evolution & Gene Duplication • Gene duplication-def- the accidental duplication of a gene on a chromosome • So how does gene duplication fit in with molecular evolution? • As long as there is a good copy of the gene it should work in the organism  this can lead to extra genetic material which can cause an organism to evolution at a molecular further along the evolutionary timeline

  26. Rates of Evolution • Gene Duplication • Ex/ hemoglobin vs. myoglobin • Mosaic Evolution-def-a change in a portion of an organism while the basic form of the organism is retained • Ex/Birds • basic body type-highly conserved • Particular parts of birds are rapidly changing – beaks, wing modification, legs

More Related